A small earthquake rumbled through San Diego on Easter Sunday one-and-a-half years ago, crumbling roadways and inflicting damage to San Diego Gas & Electric's (SDG&E's) Imperial Valley substation. Dubbed the “Mexicali Earthquake,” the event registered 7.2 on the moment magnitude scale, and it only lasted 40 seconds.

The epicenter of the earthquake was 40 miles southeast of the Imperial Valley substation in Baja California, while fault ruptures were only 9 miles from the substation. A maximum acceleration pulse of 0.29g occurred in the area, according to the U.S. Geological Survey and United Research Services Corp.

The earthquake brought down one of the utility's most important substations, which was put in service in the 1980s. In the future, as SDG&E constructs its new Sunrise Powerlink, Imperial Valley substation will serve a valuable role in the 117-mile interconnection that will carry renewable energy to San Diego. Not only have SDG&E crews replaced damaged equipment, but they are also in the process of hardening the substation with seismically qualified components.

Damage Assessment

The Imperial Valley substation, which currently features two 500/230-kV transformer banks and three 500-kV bays, is located in the middle of the desert 25 miles from El Centro, California. Because it is two-and-half hours from the operating district, SDG&E managers had to find the fastest way to the substation following the earthquake to access the damage.

To get to the site as quickly as possible, two managers traveled by helicopter. Once they arrived at the substation, they discovered that the earthquake primarily damaged the 500-kV equipment, which was larger, heavier and higher off the ground than the 230-kV components. For example, three arrestors, bus support clamps, transmission line tie-down hardware and several disconnects were broken or taken out of service in the 500-kV yard. Fortunately, SDG&E protective relaying operated as designed and took everything off-line, limiting fault duty on the major equipment.

During the Mexicali Earthquake, numerous SDG&E bus supports broke on each side, and the wire fell to the ground. As a result, it ruined some of the disconnects the wire was attached to, which had to be repaired and realigned before they were put back in service. When the bus supports were broken, they dragged the wire down with them and sent the disconnects down to the ground.

While the earthquake inflicted damage to the inside of the substation, it didn't topple over a single 500-kV transmission pole, some of which were located right along the fault line. In addition, the shaking didn't damage the substation foundations, buildings or structural steel. Some of the major substation equipment, such as the 500-kV series capacitors and 500-kV circuit breakers, suffered minimal to no damage due to previous retrofits and upgrades.

While there were aftershocks, they didn't result in any further destruction despite significant ground movement. Other equipment shifted around the substation yard due to the aftershocks. For example, the series capacitors swayed about 5 ft at the top of this structure.

Power Restoration

The earthquake damaged equipment at the Imperial Valley substation on the 500-kV transmission lines, the 500-kV buses as well as the 500/230-kV transformers. As a result, the Southwest Power Link went down, which impacted not only SDG&E, but also the electric systems of Comisión Federal de Electricidad (CFE), the Imperial Irrigation District (IID) and the California Independent System Operator (Cal-ISO).

SDG&E brought additional generation on-line and rerouted power. As a result, it was able to avoid any customer outages and immediately begin to restore operations to normal.

The field crews first worked on restoring the 500-kV transmission lines to Arizona.

The crews pulled an all-nighter to return the line to service on April 5, 2010, less than 32 hours from the time of the earthquake. The connection through the 230-kV bus was restored four days later, and the utility was able to establish a 230-kV connection from IID to CFE. Once the substation electricians were able to establish these connections, they then focused on restoring the transformers. The crew that was in charge of the restoration worked on one transformer at a time to replace the broken bushings and arrestors. Less than a month later, the utility was able to return both of its 500/230-kV transformer banks back to service.

The most significant problem that the utility faced was that it didn't have any spare bushings. As a result, the company called all of its utility contacts for emergency assistance. Within a few days, both Dominion Resources Inc. and the Bonneville Power Administration both sold SDG&E a total of five 230-kV replacement bushings. The company also acquired three 500-kV bushings from the Trench Group to replace the ones lost during the earthquake. In addition to buying components from other utilities, SDG&E also borrowed equipment from other job sites to get the Imperial Valley substation up and running.

Another key to rapid restoration was that the utility contracted with Waukesha Electric Service on the day of the earthquake. SDG&E wanted to get the banks on-line as soon as possible, yet it knew it didn't have the manpower to get the job done in a quick manner. By working with the contractor, however, the company was able to accomplish more work in a shorter time frame.

Waukesha workers helped SDG&E to pump oil from the transformers, which had rocked from east to west, in the same direction that the ground moved during the earthquake. The shaking rocked the 230-kV bushings off the pedestals, which the contract crew replaced. In addition, they had to perform work on the 500-kV bushing after a 500-kV arrestor snapped and added excessive weight to the bushing. When the bushing broke, it then lost all of its oil, so the workers also had to contain and dispose of it.

Lessons Learned

Due to the damage sustained by the earthquake, SDG&E is investing in retrofits to its Imperial Valley substation. For example, the utility is replacing 500-kV and 230-kV porcelain transformer bushings with polymer composite bushings from ABB Micafil. The new bushings are seismically qualified to the new IEEE 693 standard, “Recommended Practice for Seismic Design of Substations.” The utility is also changing all of its 500-kV porcelain lightning arrestors to polymer arrestors from Siemens. The field crews are also upgrading the utility's 500-kV tie-down hardware to an open-air spring assembly.

Another way that SDG&E is hardening its substation is by investing in more robust bus supports. Following the earthquake, a SDG&E manager talked to a New Zealand utility that had lost the same amount of equipment in a similar event, but its bus supports survived the earthquake. After further discussion, the SDG&E manager discovered that while SDG&E's bus supports were only 1 inch around, those in New Zealand were 3 inches around. By the end of 2011, the utility plans to replace the 500-kV bus supports with more robust and larger supports, according to an SDG&E report.

To ensure that the new equipment can handle an earthquake, SDG&E called upon ABB, which had tested the bushing design in its laboratory in Europe with 1 minute of shaking at 1G. In addition, Southern States is undergoing shake table testing of its 500-kV disconnects at UC Berkeley, California. SDG&E sent its bus support clamps to MacTec for mechanical strength testing and also hired Black & Veatch to confirm the mechanical design requirements of the bus supports.

SDG&E is not only swapping out its broken substation components with seismically qualified equipment, but the utility is also planning to keep spare equipment, extra bushings and more bus supports on hand for emergencies. The substation engineering division plans to work with the asset management and substation construction and maintenance departments to maintain an updated spare equipment policy.

The new spare equipment policy calls for storage of components at different facilities other than SDG&E's most seismically active substation. The utility plans to have arrestors, circuit breakers and disconnect switches on hand at its major substations. Then if another earthquake occurs, the crews can bring at least one bay into service.

The utility is also now tracking critical supplies, and a logistics center knows where all of the spare parts are at all times. SDG&E is also revamping its “bushing barn” storage area to make it more earthquake-proof. In addition, the transmission group is storing enough emergency structures for a future earthquake at the Imperial Valley substation as well.

By taking a proactive rather than a reactive approach, SDG&E is able to prepare for earthquakes and other natural disasters. The next time a catastrophic event tests the integrity of the substation components, the utility hopes that the hardened equipment will hold its own against Mother Nature.

Frank Johnson (fjohnson@semprautilities.com) is the substation construction and maintenance manager for SDG&E.

Jon Kress (jkress@semprautilities.com) is a construction superintendent with a primary responsibility for construction and maintenance. He works with the electric transmission and distribution division on the purchase of new transformers. He has been with the company since 1977.

Karl Iliev (kiliev@semprautilities.com) is the substation engineering and design manager for SDG&E and has been with the company for 10 years. He is responsible for the design and the engineering of all new and existing substations.

21 — 7,000-gal tankers with secondary oil containment
2 — 50-ton cranes with operators
4 — Large 480-V, 600-A generators
2 — 40-ft processing trailers for vacuum and oil filling
8 — Aerial man lifts
4 — SDG&E bucket trucks
15 — Heavy-duty construction trucks
100 — People who worked on the restoration

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